U.S. patent application number 15/046515 was filed with the patent office on 2016-08-04 for pseudomorphic electronic and optoelectronic devices having planar contacts.
The applicant listed for this patent is Shawn R. Gibb, James R. Grandusky, Muhammad Jamil, Mark C. Mendrick, Leo J. Schowalter. Invention is credited to Shawn R. Gibb, James R. Grandusky, Muhammad Jamil, Mark C. Mendrick, Leo J. Schowalter.
Application Number | 20160225949 15/046515 |
Document ID | / |
Family ID | 51523549 |
Filed Date | 2016-08-04 |
United States Patent
Application |
20160225949 |
Kind Code |
A1 |
Grandusky; James R. ; et
al. |
August 4, 2016 |
PSEUDOMORPHIC ELECTRONIC AND OPTOELECTRONIC DEVICES HAVING PLANAR
CONTACTS
Abstract
In various embodiments, light-emitting devices incorporate
smooth contact layers and polarization doping (i.e., underlying
layers substantially free of dopant impurities) and exhibit high
photon extraction efficiencies.
Inventors: |
Grandusky; James R.;
(Waterford, NY) ; Schowalter; Leo J.; (Latham,
NY) ; Jamil; Muhammad; (Watervliet, NY) ;
Mendrick; Mark C.; (Albany, NY) ; Gibb; Shawn R.;
(Clifton Park, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Grandusky; James R.
Schowalter; Leo J.
Jamil; Muhammad
Mendrick; Mark C.
Gibb; Shawn R. |
Waterford
Latham
Watervliet
Albany
Clifton Park |
NY
NY
NY
NY
NY |
US
US
US
US
US |
|
|
Family ID: |
51523549 |
Appl. No.: |
15/046515 |
Filed: |
February 18, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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14208379 |
Mar 13, 2014 |
9299880 |
|
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15046515 |
|
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61788141 |
Mar 15, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 33/46 20130101;
H01L 33/387 20130101; H01L 33/22 20130101; H01L 2933/0016 20130101;
H01L 33/40 20130101; H01L 33/405 20130101; H01L 2933/0058 20130101;
H01L 33/0075 20130101; H01L 33/14 20130101; H01L 33/42 20130101;
H01L 33/325 20130101; H01L 33/12 20130101; H01L 33/32 20130101;
H01L 33/06 20130101; H01L 33/60 20130101; H01L 33/04 20130101; H01L
2933/0025 20130101 |
International
Class: |
H01L 33/12 20060101
H01L033/12; H01L 33/46 20060101 H01L033/46; H01L 33/32 20060101
H01L033/32; H01L 33/40 20060101 H01L033/40; H01L 33/06 20060101
H01L033/06; H01L 33/22 20060101 H01L033/22 |
Goverment Interests
GOVERNMENT SUPPORT
[0002] This invention was made with United States Government
support under contract W911NF-09-2-0068 with the United States
Army. The United States Government has certain rights in the
invention.
Claims
1.-17. (canceled)
18. An ultraviolet (UV) light-emitting device comprising: a
substrate having an Al.sub.yGa.sub.1-yN top surface, wherein
1.0.gtoreq.y.gtoreq.0.4; a light-emitting device structure disposed
over the substrate, the device structure comprising a plurality of
layers each comprising Al.sub.xGa.sub.1-xN; an undoped graded
Al.sub.1-zGa.sub.zN layer disposed over the device structure, a
composition of the graded layer being graded in Ga concentration z
such that the Ga concentration z increases in a direction away from
the light-emitting device structure; a p-doped Al.sub.1-wGa.sub.wN
cap layer disposed over the graded layer, the p-doped
Al.sub.1-wGa.sub.wN cap layer having a Ga concentration w, wherein
1.0.gtoreq.w.gtoreq.0.8; and a metallic contact disposed over the
Al.sub.1-wGa.sub.wN cap layer and comprising at least one
metal.
19. The light-emitting device of claim 18, wherein the
Al.sub.1-wGa.sub.wN cap layer is doped with Mg.
20. The light-emitting device of claim 18, wherein the
Al.sub.1-wGa.sub.wN cap layer is at least partially relaxed.
21. The light-emitting device of claim 18, wherein the
light-emitting device has a photon extraction efficiency of greater
than 25%.
22. The light-emitting device of claim 18, wherein the graded layer
and Al.sub.1-wGa.sub.wN cap layer collectively absorb less than 80%
of UV photons generated by the light-emitting device structure and
having a wavelength less than 340 nm.
23. The light-emitting device of claim 18, wherein the at least one
metal of the metallic contact comprises Ni/Au or Pd.
24. The light-emitting device of claim 18, wherein the metallic
contact has a reflectivity to light generated by the light-emitting
device structure of approximately 60% or less.
25. The light-emitting device of claim 18, wherein the metallic
contact has a reflectivity to light generated by the light-emitting
device structure of approximately 30% or less.
26. The light-emitting device of claim 18, wherein the metallic
contact has the form of a plurality of discrete lines and/or pixels
of the at least one metal, portions of the Al.sub.1-wGa.sub.wN cap
layer not being covered by the metallic contact.
27. The light-emitting device of claim 26, further comprising a
reflector disposed over the metallic contact and the uncovered
portions of the Al.sub.1-wGa.sub.wN cap layer.
28. The light-emitting device of claim 27, wherein the reflector
comprises a metal having greater than 90% reflectivity to UV light
and a work function less than approximately 4.5 eV.
29. The light-emitting device of claim 27, wherein the reflector
has a contact resistivity to the Al.sub.1-wGa.sub.wN cap layer of
greater than approximately 5 m.OMEGA.-cm.sup.2.
30. The light-emitting device of claim 27, wherein the reflector
has a contact resistivity to the Al.sub.1-wGa.sub.wN cap layer of
greater than approximately 10 m.OMEGA.-cm.sup.2.
31. The light-emitting device of claim 27, wherein the reflector
comprises Al.
32. The light-emitting device of claim 18, wherein the
light-emitting device comprises a light-emitting diode.
33. The light-emitting device of claim 18, wherein a bottom portion
of the graded layer proximate the active device structure has a Ga
concentration z substantially equal to a Ga concentration of a
layer directly thereunder.
34. The light-emitting device of claim 18, wherein the substrate
consists essentially of doped or undoped AlN.
35. The light-emitting device of claim 18, wherein the
Al.sub.1-wGa.sub.wN cap layer has a thickness between approximately
2 nm and approximately 30 nm.
36. The light-emitting device of claim 18, wherein the
Al.sub.1-wGa.sub.wN cap layer has a surface roughness of less than
approximately 6 nm over a sample size of approximately 200
.mu.m.times.300 .mu.m.
37. The light-emitting device of claim 18, wherein the metallic
contact has a contact resistivity to the Al.sub.1-wGa.sub.wN cap
layer of less than approximately 1.0 m.OMEGA.-cm.sup.2.
Description
RELATED APPLICATION
[0001] This application claims the benefit of and priority to U.S.
Provisional Patent Application No. 61/788,141, filed Mar. 15, 2013,
the entire disclosure of which is hereby incorporated herein by
reference.
TECHNICAL FIELD
[0003] In various embodiments, the present invention relates to
improving carrier injection efficiency (e.g., the hole injection
efficiency) into high-aluminum-content electronic and
optoelectronic devices. Embodiments of the present invention also
relate to improving ultraviolet optoelectronic devices fabricated
on nitride-based substrates, in particular to improving light
extraction therefrom.
BACKGROUND
[0004] The output powers, efficiencies, and lifetimes of
short-wavelength ultraviolet light-emitting diodes (UV LEDs)--i.e.,
LEDs that emit light at wavelengths less than 350 nm--based on the
nitride semiconductor system remain limited due to high defect
levels in the active region. These limitations are particularly
problematic (and notable) in devices designed to emit at
wavelengths less than 280 nm. Particularly in the case of devices
formed on foreign substrates, such as sapphire, defect densities
remain high despite significant efforts to reduce them. These high
defect densities limit both the efficiency and the reliability of
devices grown on such substrates.
[0005] The recent introduction of low-defect, crystalline aluminum
nitride (AlN) substrates has the potential to dramatically improve
nitride-based optoelectronic semiconductor devices, particularly
those having high aluminum concentration, due to the benefits of
having lower defects in the active regions of these devices. For
example, UV LEDs pseudomorphically grown on AlN substrates have
been demonstrated to have higher efficiencies, higher power, and
longer lifetimes compared to similar devices formed on other
substrates. Generally, these pseudomorphic UV LEDs are mounted for
packaging in a "flip-chip" configuration, where the light generated
in the active region of the device is emitted through the AlN
substrate, while the LED dies have their front surfaces (i.e., the
top surfaces of the devices during epitaxial growth and initial
device fabrication prior to bonding) bonded to a patterned submount
which is used to make electrical and thermal contact to the LED
chip. A good submount material is polycrystalline (ceramic) AlN
because of the relatively good thermal expansion match with the AlN
chip and because of the high thermal conductivity of this material.
Due to the high crystalline perfection that is achievable in the
active device region of such devices, internal efficiencies greater
than 60% have been demonstrated.
[0006] Unfortunately, the photon-extraction efficiency is often
still very poor in these devices, ranging from about 4% to about
15% achieved using surface-patterning techniques--much lower than
exhibited by many visible-light (or "visible") LEDs. Thus, the
current generation of short-wavelength UV LEDs has low wall-plug
efficiencies (WPEs) of, at best, only a few percent, where the WPE
is defined as the ratio of usable optical power (in this case,
emitted UV light) achieved from the diode to the electrical power
supplied into the device. The WPE of an LED may be calculated by
taking the product of the electrical efficiency (.eta..sub.el), the
photon extraction efficiency (.eta..sub.ex), and the internal
efficiency (IE); i.e.,
WPE=.eta..sub.el.times..eta..sub.ex.times.IE. The IE itself is the
product of current injection efficiency (.eta..sub.inj) and the
internal quantum efficiency (IQE); i.e.,
IE=.eta..sub.inj.times.IQE. Thus, a low .eta..sub.ex will
deleteriously impact the WPE even after the IE has been improved
via the reduction of internal crystalline defects enabled by, e.g.,
the use of the AlN substrates referenced above as platforms for the
devices.
[0007] There are several possible contributors to low
photon-extraction efficiency. For example, currently available AlN
substrates generally have some absorption in the UV wavelength
range, even at wavelengths longer than the band edge in AlN (which
is approximately 210 nm). This absorption tends to result in some
of the UV light generated in the active area of the device being
absorbed in the substrate, hence diminishing the amount of light
emitted from the substrate surface. However, this loss mechanism
may be mitigated by thinning the AN as described in U.S. Pat. No.
8,080,833 ("the '833 patent," the entire disclosure of which is
incorporated by reference herein) and/or by reducing the absorption
in the AN substrate as described in U.S. Pat. No. 8,012,257 (the
entire disclosure of which is incorporated by reference herein).
Additionally, UV LEDs typically suffer because approximately 50% of
the generated photons are directed toward the p-contact, which
typically includes photon-absorbing p-GaN. Even when photons are
directed toward the AN surface, only about 9.4% typically escape
from an untreated surface due to the large index of refraction of
the AlN, which results in a small escape cone. These losses are
multiplicative and the average photon extraction efficiency may be
quite low.
[0008] As demonstrated in a recent publication by Grandusky et al.
(James R. Grandusky et al., 2013 Appl. Phys. Express, Vol. 6, No.
3, 032101, hereinafter referred to as "Grandusky 2013," the entire
disclosure of which is incorporated by reference herein), it is
possible to increase the photon extraction efficiency to
approximately 15% in pseudomorphic UV LEDs grown on AN substrates
via the attachment of an inorganic (and typically rigid) lens
directly to the LED die via a thin layer of an encapsulant (e.g.,
an organic, UV-resistant encapsulant compound). This encapsulation
approach, which is also detailed in U.S. patent application Ser.
No. 13/553,093, filed on Jul. 19, 2012 ("the '093 application," the
entire disclosure of which is incorporated by reference herein),
increases the critical angle of total internal reflection through
the top surface of the semiconductor die, which significantly
improves photon-extraction efficiency for the UV LEDs. In addition,
and as mentioned above, the photon extraction efficiency may be
increased by thinning the AN substrate and by roughening the
surface of the AlN substrate surface as discussed in the '833
patent.
[0009] Unfortunately, none of these efforts addresses the major
loss of photons due to absorption in the p-GaN utilized for the
p-contact to these devices. In the type of pseudomorphic UV device
described by Grandusky 2013, p-GaN is used to make the p-contact to
the LED because it allows a relatively low resistance contact to be
made to the p-side of the device. However, the band gap energy of
GaN is only 3.4 eV, and thus it is highly absorbing to photons with
wavelengths shorter than 365 nm. Since typically 50% of the photons
generated are directed toward the p-contact, these photons are
typically immediately lost due to absorption in the p-GaN. In
addition, even photons directed toward the emission surface of the
diode will typically only have a single chance to escape since, if
they are reflected back into the diode, they will likely be
absorbed by the p-GaN. The p-GaN is utilized conventionally because
it is very difficult to make a low-resistivity contact to
p-Al.sub.xGa.sub.1-xN where x is greater than 0.3. In addition,
metals that allow low-resistivity contact to the p-type nitride
semiconductor material are generally poor reflectors. This
reflectivity problem is particularly exacerbated when the desired
wavelength of the LED is less than 340 nm since most common metals
will start to absorb strongly in that regime.
[0010] In addition, prior work has suggested using a thick p-GaN
layer (or p-Al.sub.xGa.sub.1-xN layer with x<0.2) so that the
hole current spreads sufficiently from and beneath the p-metal
contacts. This approach generally will not work for devices
emitting light of wavelengths shorter than 300 nm because of the
high absorption of the p-GaN or p-Al.sub.xGa.sub.1-xN material at
these shorter wavelengths.
[0011] Alternatively, the above-referenced shortcomings might be
remedied via the use of a non-absorbing p-type semiconductor on the
p-side of the LED and the use of p-contact metallurgy that reflects
the UV photons. However, conventional approaches are unsuited to
pseudomorphic UV LEDs since these approaches use multiple layers of
thin p-Al.sub.xGa.sub.1-xN where the p-type Al.sub.xGa.sub.1-xN
layers are thin enough to be optically transparent to the UV
radiation at wavelengths shorter than 300 nm. This type of
multi-layer structure is very difficult to grow on a pseudomorphic
device structure (where the underlying substrate is either AlN or
Al.sub.xGa.sub.10xN with x>0.6), because the large amount of
strain (due to the lattice mismatch) typically causes the thin GaN
(or low aluminum content Al.sub.xGa.sub.1-xN) to island and become
very rough. In the Grandusky 2013 paper, contact roughening is
addressed by making the p-type GaN layer quite thick; however, such
layers, as detailed above, absorb UV photons and diminish UV LED
device efficiencies.
[0012] Therefore, in view of the foregoing, there is a need for
improved contact metallurgy and performance for UV LEDs,
particularly those UV LEDs produced on AlN substrates, in order to
improve characteristics, such as the WPE, of such devices.
SUMMARY
[0013] In various embodiments of the present invention, a smooth
p-GaN (or p-Al.sub.xGa.sub.1-xN layer where x<0.3) layer is
produced on the active region (e.g., a pseudomorphic active region)
of an electronic or optoelectronic device grown on a single-crystal
AN substrate or single-crystal Al.sub.xGa.sub.1-xN substrate where
x>0.6. This smooth p-GaN or p-Al.sub.xGa.sub.1-xN layer where
x<0.3 will hereinafter be abbreviated as the SPG layer. The SPG
layer is very desirable for improved fabrication of any
pseudomorphic electronic or optoelectronic device utilizing a
p-contact because it minimizes or substantially eliminates the
rough surfaces that are difficult to etch and metallize uniformly.
In various embodiments of the present invention, the SPG layer may
also be made sufficiently thin to be transparent to UV radiation
having wavelengths shorter than 340 nm. The thin, UV-transparent
SPG layer may be combined with a reflective metal contact to the
SPG layer, and this bilayer structure may then be used to both
efficiently inject holes into a UV optoelectronic device and
reflect UV photons from the p-contact. In various embodiments of
the present invention, the thin, UV-transparent SPG layer, when
combined with an appropriately designed UV reflective contact, will
allow a pseudomorphic UV LED to be fabricated on an AlN (or
Al.sub.xGa.sub.1-xN substrate with x>0.6) substrate with a
photon extraction efficiency greater than 25%. The thin SPG layer
on a pseudomorphic UV LED may be combined with a reflector metal
contact to achieve a WPE greater than 10% at wavelengths shorter
than 275 nm at current densities exceeding 30 A/cm.sup.-2.
[0014] In further embodiments of the present invention, a first
metal layer capable of making a low-resistivity contact to the SPG
layer is disposed on the SPG layer and patterned. The resulting
gaps in the first metal layer may then be filled via the deposition
of a second metal layer that is an efficient reflector of UV
photons. In this manner, the two-metal structure provides the dual
advantages of low contact resistance and high reflectivity, both of
which improve the performance of UV LEDs.
[0015] In an exemplary embodiment, Al may be used as the reflector
metal, as it has >90% reflectivity to light having a wavelength
of approximately 265 nm. However, Al is quite poor for making a
low-resistivity contact to p-type GaN or p-type Al.sub.xGa.sub.1-xN
because of its low work function (4.26 eV). The high resistivity of
the Al/nitride interface is addressed by the regions of the
low-resistivity contact metal; however, in order to prevent
absorption of the UV photons by the contact metal, preferred
embodiments of the invention utilize only limited contact areas
between the contact metal and the underlying semiconductor rather
than a contact metal-semiconductor contact area covering
substantially all of the semiconductor surface. For example, in
some embodiments (i) more than about 10% of the semiconductor
surface is covered by the contact metal, but (ii) less than about
70%, less than about 60%, less than about 50%, or even less than
40% of the semiconductor surface is covered by the contact metal,
while the remaining portion of the semiconductor surface is covered
by the reflector metal to minimize deleterious absorption of the UV
light.
[0016] In one aspect, embodiments of the invention feature a method
of forming a contact to a UV light-emitting device. A substrate
having an Al.sub.yGa.sub.1-yN top surface is provided, where
y.gtoreq.0.4 (and .ltoreq.1.0). The substrate may be substantially
entirely composed of the Al.sub.yGa.sub.1-yN material (e.g., AlN),
or the substrate may include or consist essentially of a different
material (e.g., silicon carbide, silicon, and/or sapphire) with the
Al.sub.yGa.sub.1-yN material formed thereover by e.g., epitaxial
growth; such material may be substantially fully lattice relaxed
and may have a thickness of, e.g., at least 1 .mu.m. An active,
light-emitting device structure is formed over the substrate, the
device structure including or consisting essentially of a plurality
of layers each including or consisting essentially of
Al.sub.xGa.sub.1-xN. An undoped graded Al.sub.1-zGa.sub.zN layer is
formed over the device structure, a composition of the graded layer
being graded in Ga concentration z such that the Ga concentration z
increases in a direction away from the light-emitting device
structure. (For example, the Ga concentration z may increase from a
composition of approximately 0.15 proximate the device structure to
a composition of approximately 1 at the top of the graded layer.) A
p-doped Al.sub.1-wGa.sub.wN cap layer is formed over the graded
layer, the cap layer (i) having a thickness between approximately 2
nm and approximately 30 nm, (ii) a surface roughness of less than
approximately 6 nm over a sample size of approximately 200
.mu.m.times.300 .mu.m, and (iii) a Ga concentration w.gtoreq.0.8. A
metallic contact comprising at least one metal is formed over the
Al.sub.1-wGa.sub.wN cap layer, the metallic contact having a
contact resistivity to the Al.sub.1-wGa.sub.wN cap layer of less
than approximately 1.0 m.OMEGA.-cm.sup.2.
[0017] Embodiments of the invention may include one or more of the
following in any of a variety of combinations. Forming the
Al.sub.1-wGa.sub.wN cap layer may include or consist essentially of
epitaxial growth at a temperature between 850.degree. C. and
900.degree. C. and a growth pressure less than 50 Torr, e.g.,
between approximately 10 Torr and approximately 30 Torr, for
example 20 Torr. The Al.sub.1-wGa.sub.wN cap layer may be doped
with Mg and/or may be at least partially relaxed. The
light-emitting device may have a photon extraction efficiency of
greater than 25%. The graded layer and Al.sub.1-wGa.sub.wN cap
layer may collectively absorb less than 80% of UV photons generated
by the light-emitting device structure and having a wavelength less
than 340 nm. The at least one metal of the metallic contact may
include or consist essentially of Ni/Au and/or Pd. The metallic
contact may have a reflectivity to light generated by the
light-emitting device structure of approximately 60% or less, or
even approximately 30% or less. The metallic contact may be formed
as a plurality of discrete lines and/or pixels of the at least one
metal, portions of the Al.sub.1-wGa.sub.wN cap layer not being
covered by the metallic contact. A reflector may be formed over the
metallic contact and the uncovered portions of the
Al.sub.1-wGa.sub.wN cap layer. The reflector may include or consist
essentially of a metal having greater than 60%, or even greater
than 90%, reflectivity to UV light and a work function less than
approximately 4.5 eV. The reflector may have a contact resistivity
to the Al.sub.1-wGa.sub.wN cap layer of greater than approximately
5 m.OMEGA.-cm.sup.2, or even greater than approximately 10
m.OMEGA.-cm.sup.2. The reflector may include or consist essentially
of Al.
[0018] The light-emitting device may include or consist essentially
of a light-emitting diode or a laser. A bottom portion of the
graded layer proximate the active device structure may have a Ga
concentration z substantially equal to a Ga concentration of a
layer directly thereunder, and/or a top portion of the graded layer
opposite the bottom portion of the graded layer may have a Ga
concentration z of approximately 1. Forming the Al.sub.1-wGa.sub.wN
cap layer may include or consist essentially of epitaxial growth at
a growth rate between 0.5 nm/min and 5 nm/min. Between forming the
graded layer and forming the Al.sub.1-wGa.sub.wN cap layer, a
surface of the graded layer may be exposed to a precursor of the
p-type dopant of the cap layer without exposure to a Ga precursor.
The p-type dopant of the cap layer may include or consist
essentially of Mg. The substrate may consist essentially of doped
or undoped AlN.
[0019] In another aspect, embodiments of the invention feature a UV
light-emitting device including or consisting essentially of a
substrate having an Al.sub.yGa.sub.1-yN top surface, where
y.gtoreq.0.4 (and .ltoreq.1.0), a light-emitting device structure
disposed over the substrate, the device structure including or
consisting essentially of a plurality of layers each including or
consisting essentially of Al.sub.xGa.sub.1-xN, an undoped graded
Al.sub.1-zGa.sub.zN layer disposed over the device structure, a
composition of the graded layer being graded in Ga concentration z
such that the Ga concentration z increases in a direction away from
the light-emitting device structure, a p-doped Al.sub.1-wGa.sub.wN
cap layer disposed over the graded layer, the p-doped
Al.sub.1-wGa.sub.wN cap layer (i) having a thickness between
approximately 2 nm and approximately 30 nm, (ii) a surface
roughness of less than approximately 6 nm over a sample size of
approximately 200 .mu.m.times.300 .mu.m, and (iii) a Ga
concentration w.gtoreq.0.8, and a metallic contact disposed over
the Al.sub.1-wGa.sub.wN cap layer and including or consisting
essentially of at least one metal, the metallic contact having a
contact resistivity to the Al.sub.1-wGa.sub.wN cap layer of less
than approximately 1.0 m.OMEGA.-cm.sup.2. The substrate may be
substantially entirely composed of the Al.sub.yGa.sub.1-yN material
(e.g., AlN), or the substrate may include or consist essentially of
a different material (e.g., silicon carbide, silicon, and/or
sapphire) with the Al.sub.yGa.sub.1-yN material formed thereover by
e.g., epitaxial growth; such material may be substantially fully
lattice relaxed and may have a thickness of, e.g., at least 1
.mu.m.
[0020] Embodiments of the invention may include one or more of the
following in any of a variety of combinations. The
Al.sub.1-wGa.sub.wN cap layer may be doped with Mg and/or may be at
least partially relaxed. The light-emitting device may have a
photon extraction efficiency of greater than 25%. The graded layer
and Al.sub.1-wGa.sub.wN cap layer may collectively absorb less than
80% of UV photons generated by the light-emitting device structure
and having a wavelength less than 340 nm. The at least one metal of
the metallic contact may include or consist essentially of Ni/Au
and/or Pd. The metallic contact may have a reflectivity to light
generated by the light-emitting device structure of approximately
60% or less, or even approximately 30% or less.
[0021] The metallic contact may have the form of a plurality of
discrete lines and/or pixels of the at least one metal, portions of
the Al.sub.1-wGa.sub.wN cap layer not being covered by the metallic
contact. A reflector may be disposed over the metallic contact and
the uncovered portions of the Al.sub.1-wGa.sub.wN cap layer. The
reflector may include or consist essentially of a metal having
greater than 60%, or even greater than 90%, reflectivity to UV
light and a work function less than approximately 4.5 eV. The
reflector may have a contact resistivity to the Al.sub.1-wGa.sub.wN
cap layer of greater than approximately 5 m.OMEGA.-cm.sup.2, or
even greater than approximately 10 m.OMEGA.-cm.sup.2. The reflector
may include or consist essentially of Al. The light-emitting device
may include or consist essentially of a light-emitting diode or a
laser. A bottom portion of the graded layer proximate the active
device structure may have a Ga concentration z substantially equal
to a Ga concentration of a layer directly thereunder, and/or a top
portion of the graded layer opposite the bottom portion of the
graded layer may have a Ga concentration z of approximately 1. The
substrate may consist essentially of doped or undoped AlN. These
and other objects, along with advantages and features of the
present invention herein disclosed, will become more apparent
through reference to the following description, the accompanying
drawings, and the claims. Furthermore, it is to be understood that
the features of the various embodiments described herein are not
mutually exclusive and may exist in various combinations and
permutations. As used herein, the term "substantially"
means.+-.10%, and in some embodiments, .+-.5%. The term "consists
essentially of" means excluding other materials that contribute to
function, unless otherwise defined herein. Nonetheless, such other
materials may be present, collectively or individually, in trace
amounts.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] In the drawings, like reference characters generally refer
to the same parts throughout the different views. Also, the
drawings are not necessarily to scale, emphasis instead generally
being placed upon illustrating the principles of the invention. In
the following description, various embodiments of the present
invention are described with reference to the following drawings,
in which:
[0023] FIG. 1 is an optical profilometry surface-roughness scan of
a conventional contact layer for an LED device;
[0024] FIG. 2 is an optical profilometry surface-roughness scan of
a contact layer for a light-emitting device in accordance with
various embodiments of the invention;
[0025] FIGS. 3A and 3B are schematic cross-sections of
light-emitting devices in accordance with various embodiments of
the invention;
[0026] FIG. 4A is an atomic force microscopy scan of a capping
layer for a light-emitting device in accordance with various
embodiments of the invention;
[0027] FIG. 4B is an atomic force microscopy scan of a conventional
capping layer for a light-emitting device; and
[0028] FIG. 5 is a schematic cross-section of a portion of a
light-emitting device in accordance with various embodiments of the
invention.
DETAILED DESCRIPTION
[0029] Embodiments of the invention include pseudomorphic
Al.sub.xGa.sub.1-xN electronic and light-emitting devices on a
substrate having an Al.sub.yGa.sub.1-yN top surface, where
y.gtoreq.0.4 (and .ltoreq.1.0). The substrate may be substantially
entirely composed of the Al.sub.yGa.sub.1-yN material (e.g., AlN),
or the substrate may include or consist essentially of a different
material (e.g., silicon carbide, silicon, and/or sapphire) with the
Al.sub.yGa.sub.1-yN material formed thereover by e.g., epitaxial
growth; such material may be substantially fully lattice relaxed
and may have a thickness of, e.g., at least 1 .mu.m. (Although
light-emitting devices in accordance with preferred embodiments of
the present invention are configured for the emission of UV light,
the substrate need not be transparent to UV radiation (e.g.,
silicon), since it may be partially or substantially removed during
device fabrication.) The devices according to embodiments of the
invention also have a thin p-GaN or p-Al.sub.xGa.sub.1-xN contact
layer that is smooth (i.e., having a root-mean-square (Rq) surface
roughness of less than approximately 6 nm, or even less than
approximately 1 nm). The roughness may characterized with optical
profilometry over a sample size of approximately 200
.mu.m.times.300 .mu.m, e.g., 233 .mu.m.times.306.5 .mu.m. FIG. 1
depicts a profilometry scan of a conventional rough contact-layer
surface having an Rq value of approximately 33 nm. In contrast,
FIG. 2 depicts a smooth contact surface in accordance with
embodiments of the present invention that has an Rq value of only
approximately 6 nm.
[0030] In preferred embodiments of the invention, the threading
dislocation density (TDD) in the active region of the device is
less than 10.sup.5 cm .sup.2. Furthermore, in preferred
embodiments, the thin p-GaN or p-Al.sub.xGa.sub.1-xN (SPG) final
layer will be sufficiently thin to allow light with wavelengths
shorter than 340 nm to be transmitted with minimal absorption
(i.e., absorption in a single pass no greater than 80%, no greater
than 50%, or even no greater than 40%). By decreasing the thickness
of the SPG layer or by increasing the concentration of Al in a
given thickness for the SPG layer, the UV absorption at wavelengths
shorter than 340 nm may be decreased to 50%, to 25%, to 10%, or
even to 5% or less. For example, for a UV LED designed to operate
at 265 nm, the absorption coefficient of this radiation in the
p-GaN layer will be approximately 1.8.times.10.sup.5 cm.sup.-1.
Table 1 illustrates various thickness-absorption relationships for
Al.sub.xGa.sub.1-xN layers of various Al contents x and thicknesses
for a variety of emission wavelengths. In Table 1, absorption
values are shown for layers of 40% Al only for emission wavelengths
up to 265 nm, as such layers become substantially transparent at
larger wavelengths.
TABLE-US-00001 TABLE 1 % absorbed (single pass) Al % Emission in
wavelength thickness (microns) AlGaN (nm) 0.001 0.002 0.003 0.01
0.025 0.05 0.1 0.2 0 235 2.5% 4.9% 7.2% 22.1% 46.5% 71.3% 91.8%
99.3% 0 250 2.1% 4.1% 6.1% 18.9% 40.8% 65.0% 87.8% 98.5% 0 265 1.7%
3.4% 5.1% 16.1% 35.4% 58.3% 82.6% 97.0% 0 280 1.6% 3.1% 4.7% 14.8%
33.0% 55.1% 79.8% 95.9% 0 305 1.3% 2.6% 3.8% 12.2% 27.7% 47.8%
72.7% 92.6% 20 235 1.8% 3.6% 5.4% 16.9% 37.0% 60.3% 84.3% 97.5% 20
250 1.6% 3.1% 4.7% 14.8% 33.0% 55.1% 79.8% 95.9% 20 265 1.3% 2.7%
4.0% 12.6% 28.6% 49.1% 74.1% 93.3% 20 280 1.2% 2.4% 3.5% 11.3%
25.9% 45.1% 69.9% 90.9% 20 305 0.9% 1.8% 2.7% 8.6% 20.1% 36.2%
59.3% 83.5% 40 235 1.4% 2.8% 4.1% 13.1% 29.5% 50.3% 75.3% 93.9% 40
250 1.2% 2.4% 3.5% 11.3% 25.9% 45.1% 69.9% 90.9% 40 265 1.0% 2.0%
3.0% 9.5% 22.1% 39.3% 63.2% 86.5%
[0031] In order to improve the photon extraction efficiency and
enable extraction of photons directed towards the p-type material,
a UV reflector may be introduced into the device structure to
reflect transmitted photons and direct them towards the Al
substrate so that they may be extracted from the device. In visible
LEDs, this is often accomplished by using a silver p-contact, as
silver both forms an ohmic contact to visible-LED structures and is
reflective to visible photons. In addition, the layers that form a
visible LED are generally transparent to the photons being
generated in the quantum wells. However, the reflectivity of silver
drops rapidly in the UV range. The reflectivities of most other
common metals also drop as the wavelength decreases into the UV
range with the exception of Al, which unfortunately does not form a
good ohmic contact to p-type GaN or Al.sub.xGa.sub.1-xN.
[0032] Thus, in order to reflect photons while still achieving good
ohmic contact, a fairly non-reflective (at least to UV photons)
contact metallurgy (e.g., Ni/Au or Pd) may be formed over the
contact layer but patterned to reduce the surface "footprint" of
the contact over the semiconductor. In this manner, the surface
area over the device layers that is non-reflective to UV photons is
minimized, yet good ohmic contact to the semiconductor is still
achieved. In order to reflect at least a portion of the UV photons,
a reflective metal such as Al may be provided directly over the
semiconductor between the non-reflective contact regions. The
reflective metal makes an ohmic contact with the non-reflective
metal, enabling electrical contact to the LED while utilizing the
superior metal-semiconductor contact formed by the non-reflective
metal.
[0033] In such embodiments, the SPG layer may include or consist
essentially of a p-GaN or p-Al.sub.xGa.sub.1-xN layer where
x<0.3. Typically, thicker SPG layers may be utilized as the Ga
content is decreased, as the lattice-mismatch strain (that may
roughen the SPG layer) between the SPG layer and the underlying AN
substrate decreases. However, the Ga content of the SPG layer is
preferably maintained at or above 70% in order to enable a highly
doped, low-resistivity layer.
[0034] For p-type Al.sub.xGa.sub.1-xN layers doped with Mg, as the
Al mole fraction (x) is increased, the activation energy of the Mg
impurity is increased. This leads to lower activation of the Mg,
resulting in lower hole concentration as the Al mole fraction is
increased. One solution to this is to utilize polarization-induced
doping, which may be achieved by the grading of an
Al.sub.xGa.sub.1-xN layer from high x to lower x as it is
deposited. This may be used to achieve hole concentrations much
higher than may be achieved through conventional impurity doping.
In addition, this technique may result in improved carrier
mobilities due to lack of impurity scattering and reduced
temperature dependence of the hole concentration. High hole
concentrations may be achieved in the absence of impurity doping or
in addition to impurity doping. Preferred embodiments of the
invention feature low dislocation density in pseudomorphic graded
layers, which enables high hole concentration in the absence of
impurity doping, thus allowing for higher conductivity and improved
current spreading from thin transparent layers. These high hole
concentrations make it possible to achieve p-contacts with low
resistivity. In particular, resistivities less than 10
m.OMEGA.-cm.sup.2 may be achieved in accordance with embodiments of
the present invention. In preferred embodiments, resistivities less
than 5 m.OMEGA.-cm.sup.2 are achieved and utilized in UV LEDs. For
contacts with resistivities of 10 m.OMEGA.-cm.sup.2, the device may
be operated at 30 A/cm.sup.2 with a 1:3 ratio of contact metal to
reflector metal (as detailed above) and achieve a voltage drop
across the p-contact of less than 1.2 V with a device area of
0.0033 cm.sup.2. By covering 75% of the p-contact area with good
reflector metal and using an SPG layer with absorption less than
80%, it is possible to achieve photon extraction efficiencies in UV
LEDs that are greater than 25%, particularly when combined with the
efficient photon extraction techniques described above. When the
high photon extraction efficiency of greater than 25% is combined
with the low-resistivity contact described above, embodiments of
the invention exhibit wall plug efficiencies greater than 10% at an
operating current density exceeding 30 A/cm.sup.2.
[0035] FIG. 3A depicts a pseudomorphic UV light emitting diode
("PUVLED") structure 300 in accordance with embodiments of the
present invention. A semiconductor substrate 305, which includes or
consists essentially of, e.g., a substrate having an
Al.sub.yGa.sub.1-yN top surface, where y.gtoreq.0.4 (and
.ltoreq.1.0), is provided. The substrate may be substantially
entirely composed of the Al.sub.yGa.sub.1-yN material (e.g., AlN),
or the substrate may include or consist essentially of a different
material (e.g., silicon carbide, silicon, and/or sapphire) with the
Al.sub.yGa.sub.1-yN material formed thereover by e.g., epitaxial
growth; such material may be substantially fully lattice relaxed
and may have a thickness of, e.g., at least 1 .mu.m. As mentioned
above, the substrate 305 need not be transparent to UV radiation
(e.g., silicon), since it may be partially or substantially removed
during device fabrication. Semiconductor substrate 305 may be
miscut such that the angle between its c-axis and its surface
normal is between approximately 0.degree. and approximately
4.degree.. In a preferred embodiment, the misorientation of the
surface of semiconductor substrate 305 is less than approximately
0.3.degree., e.g., for semiconductor substrates 305 that are not
deliberately or controllably miscut. In other embodiments, the
misorientation of the surface of semiconductor substrate 305 is
greater than approximately 0.3.degree., e.g., for semiconductor
substrates 305 that are deliberately and controllably miscut. In a
preferred embodiment, the direction of the miscut is towards the
a-axis. The surface of semiconductor substrate 305 may have a
group-III (e.g., Al--) polarity or N-polarity, and may be
planarized, e.g., by chemical-mechanical polishing. The RMS surface
roughness of semiconductor substrate is preferably less than
approximately 0.5 nm for a 10 .mu.m.times.10 .mu.m area. In some
embodiments, atomic-level steps are detectable on the surface when
probed with an atomic-force microscope. The threading dislocation
density of semiconductor substrate 305 may be measured using, e.g.,
etch pit density measurements after a 5 minute KOH--NaOH eutectic
etch at 450 .degree. C. Preferably the threading dislocation
density is less than approximately 2.times.10.sup.3 cm .sup.2. In
some embodiments substrate 305 has an even lower threading
dislocation density. Semiconductor substrate 305 may be topped with
a homoepitaxial layer (not shown) that includes or consists
essentially of the same semiconductor material present in
semiconductor substrate 300, e.g., AlN.
[0036] In an embodiment, an optional graded buffer layer 310 is
formed on semiconductor substrate 305. Graded buffer layer 310 may
include or consist essentially of one or more semiconductor
materials, e.g., Al.sub.xGa.sub.1-xN. In a preferred embodiment,
graded buffer layer 310 has a composition approximately equal to
that of semiconductor substrate 305 at an interface therewith in
order to promote two-dimensional growth and avoid deleterious
islanding (such islanding may result in undesired elastic strain
relief and/or surface roughening in graded buffer layer 310 and
subsequently grown layers). The composition of graded buffer layer
310 at an interface with subsequently grown layers (described
below) is generally chosen to be close to (e.g., approximately
equal to) that of the desired active region of the device (e.g.,
the Al.sub.xGa.sub.1-xN concentration that will result in the
desired wavelength emission from the PUVLED). In an embodiment,
graded buffer layer 310 includes Al.sub.xGa.sub.1-xN graded from an
Al concentration x of approximately 100% to an Al concentration x
of approximately 60%.
[0037] A bottom contact layer 320 is subsequently formed above
substrate 305 and optional graded layer 310, and may include or
consist essentially of Al.sub.xGa.sub.1-xN doped with at least one
impurity, e.g., Si. In an embodiment, the Al concentration x in
bottom contact layer 320 is approximately equal to the final Al
concentration x in graded layer 310 (i.e., approximately equal to
that of the desired active region (described below) of the device).
Bottom contact layer 320 may have a thickness sufficient to prevent
current crowding after device fabrication (as described below)
and/or to stop on during etching to fabricate contacts. For
example, the thickness of bottom contact layer 320 may be less than
approximately 200 nm. When utilizing a bottom contact layer 320 of
such thickness, the final PUVLED may be fabricated with back-side
contacts. In many embodiments, bottom contact layer 320 will have
high electrical conductivity even with a small thickness due to the
low defect density maintained when the layer is pseudomorphic. As
utilized herein, a pseudomorphic film is one where the strain
parallel to the interface is approximately that needed to distort
the lattice in the film to match that of the substrate. Thus, the
parallel strain in a pseudomorphic film will be nearly or
approximately equal to the difference in lattice parameters between
an unstrained substrate parallel to the interface and an unstrained
epitaxial layer parallel to the interface.
[0038] A multiple-quantum well ("MQW") layer 330 is fabricated
above bottom contact layer 320. MQW layer 330 corresponds to the
"active region" of PUVLED structure 300 and includes a plurality of
quantum wells, each of which may include or consist essentially of
AlGaN. In an embodiment, each period of MQW layer 330 includes an
Al.sub.xGa.sub.1-xN quantum well and an Al.sub.yGa.sub.1-yN
barrier, where x is different from y. In a preferred embodiment,
the difference between x and y is large enough to obtain good
confinement of the electrons and holes in the active region, thus
enabling high ratio of radiative recombination to non-radiative
recombination. In an embodiment, the difference between x and y is
approximately 0.05, e.g., x is approximately 0.35 and y is
approximately 0.4. However, if the difference between x and y is
too large, e.g., greater than approximately 0.3, deleterious
islanding may occur during formation of MQW layer 330. MQW layer
330 may include a plurality of such periods, and may have a total
thickness less than approximately 50 nm. Above MQW layer 330 may be
formed an optional thin electron-blocking (or hole-blocking if the
n-type contact is put on top of the device) layer 340, which
includes or consists essentially of, e.g., Al.sub.xGa.sub.1-xN,
which may be doped with one or more impurities such as Mg.
Electron-blocking layer 340 has a thickness that may range between,
e.g., approximately 10 nm and approximately 50 nm. A top contact
layer 350 is formed above electron blocking layer 340, and includes
or consists essentially of one or more semiconductor materials,
e.g., Al.sub.xGa.sub.1-xN, doped with at least one impurity such as
Mg. Top contact layer 350 is doped either n-type or p-type, but
with conductivity opposite that of bottom contact layer 310. The
thickness of top contact layer 350 is, e.g., between approximately
50 nm and approximately 100 nm. Top contact layer 350 is capped
with a cap layer 360, which includes or consists essentially of one
or more semiconductor materials doped with the same conductivity as
top contact layer 350. In an embodiment, cap layer 360 includes GaN
doped with Mg, and has a thickness between approximately 10 nm and
approximately 200 nm, preferably approximately 50 nm. In some
embodiments, high-quality ohmic contacts may be made directly to
top contact layer 350 and cap layer 360 is omitted. In other
embodiments, top contact layer 350 and/or electron-blocking layer
340 are omitted and the top contact is formed directly on cap layer
360 (in such embodiments, cap layer 360 may be considered to be a
"top contact layer"). While it is preferred that layers 310-340 are
all pseudomorphic, top contact layer 350 and/or cap layer 360 may
relax without introducing deleterious defects into the active
layers below which would adversely affect the performance of PUVLED
structure 300 (as described below with reference to FIG. 3B). Each
of layers 310-350 is pseudomorphic, and each layer individually may
have a thickness greater than its predicted critical thickness.
Moreover, the collective layer structure including layers 310-350
may have a total thickness greater than the predicted critical
thickness for the layers considered collectively (i.e., for a
multiple-layer structure, the entire structure has a predicted
critical thickness even when each individual layer would be less
than a predicted critical thickness thereof considered in
isolation).
[0039] In various embodiments, layers 310-340 of PUVLED structure
300 are pseudomorphic, and cap layer 360 is intentionally relaxed.
As shown in FIG. 3B, layers 310-340 are formed as described above
with reference to FIG. 3A. Cap layer 360 is subsequently formed in
a partially or substantially strain-relaxed state via judicious
selection of its composition and/or the deposition conditions. For
example, the lattice mismatch between cap layer 360 and substrate
305 and/or MQW layer 330 may be greater than approximately 1%,
greater than approximately 2%, or even greater than approximately
3%. In a preferred embodiment, cap layer 360 includes or consists
essentially of undoped or doped GaN, substrate 305 includes or
consists essentially of AlN, and MQW layer 330 includes or consists
essentially of multiple Al.sub.0.55Ga.sub.0.45N quantum wells
interleaved with Al.sub.0.75Ga.sub.0.25N barrier layers, and cap
layer 360 is lattice mismatched by approximately 2.4%. Cap layer
360 may be substantially relaxed, i.e., may have a lattice
parameter approximately equal to its theoretical unstrained lattice
constant. As shown, a partially or substantially relaxed cap layer
360 may contain strain-relieving dislocations 370 having segments
threading to the surface of cap layer 360 (such dislocations may be
termed "threading dislocations"). The threading dislocation density
of a relaxed cap layer 360 may be larger than that of substrate 305
and/or layers 310-340 by, e.g., one, two, or three orders of
magnitude, or even larger. Cap layer 360 is preferably not formed
as a series of coalesced or uncoalesced islands, as such islanding
may deleteriously impact the surface roughness of cap layer
360.
[0040] A graded layer may be formed between layers 310-340 and cap
layer 360, and its composition at its interfaces with layers 340,
360 may substantially match the compositions of those layers. The
thickness of this graded layer, which is preferably
pseudomorphically strained, may range between approximately 10 nm
and approximately 50 nm, e.g., approximately 30 nm. In some
embodiments, epitaxial growth may be temporarily stopped between
growth of the graded layer and cap layer 360.
[0041] In an exemplary embodiment, an electron-blocking layer 340
including or consisting essentially of Al.sub.0.8Ga.sub.0.2N or
Al.sub.0.85Ga.sub.0.15N is formed over MQW layer 330. Prior to
formation of cap layer 360 including or consisting essentially of
GaN, a graded layer is formed over electron-blocking layer 340. The
graded layer may be graded in composition from, for example,
Al.sub.0.85Ga.sub.0.15N to GaN over a thickness of approximately 30
nm. The graded layer may be formed by, e.g., MOCVD, and in this
embodiment is formed by ramping the flow of TMA and TMG (by ramping
the flow of hydrogen through their respective bubblers) from the
conditions utilized to form electron-blocking layer 340 to 0
standard cubic centimeters per minute (sccm) and 6.4 sccm,
respectively, over a period of approximately 24 minutes, thus
resulting in a monotonic grade from Al.sub.0.85Ga.sub.0.15N to GaN
(all of the other growth conditions are substantially fixed). The
thickness of the graded layer in this exemplary embodiment is
approximately 30 nm, and a hole concentration of approximately
3.times.10.sup.19 cm.sup.-3 may be achieved through polarization
doping without impurity doping (e.g., even being substantially free
of doping impurities), as modeled using SiLENSe software. In
general, polarization doping is enabled by the polarization in
nitride materials that is due to the difference in
electronegativity between the metal atoms and the nitrogen atoms.
This results in a polarization field along asymmetric directions in
the wurtzite crystal structure. In addition, strain in the layers
may result in additional piezoelectric polarization fields and thus
additional polarization doping. These fields create fixed charges
at abrupt interfaces (e.g., two-dimensional sheets) or graded
composition layers (e.g., three-dimensional volumes), which results
in mobile carriers of the opposite sign. The magnitude of the total
charge is defined by the difference in Al compositions within the
graded layer, i.e., the difference between the starting composition
and the final composition. The concentration of carriers is defined
by the total charge divided by the graded layer thickness. A very
high carrier concentration may be achieved by a high composition
change over a small thickness, while a lower composition change or
larger grading thickness typically results in a smaller carrier
concentration; however, for a given composition change the total
number of carriers is generally constant.
[0042] As detailed above, preferred embodiments of the present
invention utilize very thin SPG layers in order to minimize
absorption of UV photons therein. Such SPG layers preferably have
thicknesses of less than 50 nm, e.g., between approximately 10 nm
and approximately 30 nm. In an embodiment, smooth (25-50 nm) p-GaN
layers were grown on a typical pseudomorphic LED structure
(AlN/n-AlGaN/MQW/electron-blocking layer/p-GaN) by MOCVD and
trimethylgallium (TMGa) and NH.sub.3 were used as Ga and N
precursors. Some conventional p-GaN layers are grown at
1000.degree. C. and at a pressure of 100 Torr, and often these
layers are rough, exhibiting an islanded or pyramidal morphology.
Such approaches are encouraged by the conventional wisdom in the
art, which indicates that one should enhance the mobility of the Ga
adatom to promote lateral growth and coalescence of the layer.
Thus, conventional wisdom teaches that contact-layer growth should
use increased V/III ratios and higher temperatures. However, such
techniques were unable to achieve smooth surfaces on the
pseudomorphic layer in the thickness range utilized in embodiments
of the present invention. Notably, the large strain in the
pseudomorphic layer enhances island formation and increased surface
roughness. Unexpectedly, in order to suppress such surface
roughening, in accordance with embodiments of the present
invention, growth temperatures of 850.degree. C.-900.degree. C. may
be utilized for growth of the SPG layer, and growth pressures of 20
Torr may be utilized to enhance the adatom mobility at this lower
growth-temperature regime. The growth rate of smooth p-GaN is only
approximately 5 nm/min. The morphological and elemental properties
of resulting SPG layers were investigated using atomic force
microscopy (AFM) and secondary ion mass spectroscopy (SIMS). AFM
shows smoother p-GaN layers (Rq value of approximately 0.85 nm) as
shown in FIG. 4A, compared to the rougher morphology of
conventional p-GaN (Rq value of approximately 7.2 nm) shown in FIG.
4B. Here, the actual island heights are over 50 nm and these
thicker islands result in higher absorption and also leave areas
uncovered by p-GaN which will result in poor p-contact by the
contact metallization when these holes occur in the regions that
are covered by the contact metallization. SIMS analysis shows
higher doping concentration (by a factor of two) in the smooth
p-GaN compared to the conventional p-GaN; however, the
concentration is not constant and does not reach equilibrium until
growth of .about.25 nm of p-GaN, resulting in difficulties making
ohmic contacts to layers thinner than 25 nm. In order to overcome
this issue, a soak, i.e., exposure within the deposition chamber,
(of, e.g., 1-10 minutes, for example 5 minutes) with only the
dopant (e.g., Mg) source (i.e., no Ga source) flowing may be
utilized to saturate the surface prior to growth initiation. For
example, bis-cyclopentadienylmagnesium (Cp2Mg) may be utilized at
an Mg source for the soak when MOCVD is being utilized for layer
growth. The precursor may be disposed within a bubbler, and a
carrier gas such as nitrogen or hydrogen may be flowed into the
bubbler to form a gas solution saturated with the dopant precursor.
This enables higher dopant concentration and good ohmic contact
formation to layers as thin as 5 nm. In summary, very thin p-GaN
layers (<10 nm) may be easily realized in this growth regime
owing to the slower growth rate and the conformal morphology while
the doping concentrations may be optimized by adjusting the input
precursor flows.
[0043] In an exemplary embodiment, polarization doping and a thin
SPG layer are combined with a patterned reflector as shown in FIG.
5, which depicts a portion of a UV LED device 500. In device 500,
region 510 includes or consists essentially of the AN substrate and
the active region of the device, for example as detailed above and
illustrated in FIG. 3A. Region 510 is topped with a SPG layer 520,
which is kept smooth to enable a very thin layer with high UV
transparency. A contact layer 530, formed on the SPG layer 520, is
typically substantially not UV reflective but forms a good ohmic
contact to the SPG layer 520. In an exemplary embodiment, contact
layer 530 includes or consists essentially of Ni/Au. As shown, the
contact layer is, in preferred embodiments, patterned onto the
surface of SPG layer 520. The spacing between individual portions
of contact layer 530 may be defined via, e.g., conventional
photolithography. The pattern may be in the form of lines or
patterns of isolated "pixels" (or "islands") as shown in FIG. 5.
Lines may have widths of, for example, 1 .mu.m to 50 .mu.m, e.g., 5
.mu.m, and may have spacings therebetween of, for example 1 .mu.m
to 50 .mu.m, e.g., 5 .mu.m. Pixels may be, for example,
substantially cubic or rectangular solids or may even be
substantially hemispherical, and pixels may have a dimension such
as width, length, or diameter of, for example, 1 .mu.m to 50 .mu.m,
e.g., 5 .mu.m. The contact area and the spacing are typically
defined to optimize the wall plug efficiency of the device.
[0044] As shown in FIG. 5, the contact layer 530 may be capped with
a reflector 540 formed both above the contact layer 530 (or
isolated portions thereof) and between portions of the contact
layer 530 (i.e., in direct contact with SPG layer 520). The
reflector 540 typically includes or consists essentially of a metal
(or metal alloy) that is highly reflective to UV light but that
does not form a good ohmic contact to the SPG layer 520. For
example, the reflector 540 may include or consist essentially of
Al. The contact area of the contact layer 530 will generally
determine, at least in part, the effective contact resistance of
the combined contact layer 530 and reflector 540. For instance, if
10% of the area is covered by the contact layer 530, then the
effective contact resistance is increased by a factor of ten.
However, at the same time, the reflector area (i.e., the area of
SPG layer 520 capped directly by reflector 540, without contact
layer 530 therebetween) is increased. In an exemplary embodiment,
the contact resistivity of the contact layer 530 is less than
approximately 1.0 m.OMEGA.-cm.sup.2, or even less than
approximately 0.5 m.OMEGA.-cm.sup.2. By using a 1:10 ratio of
contact 530 area to reflector 540 area, the effective contact
resistance is increased to 5 m.OMEGA. and the effective (averaged
over all area) reflector is reduced by 10% (e.g., a 90%
reflectivity of the reflector 540 is effectively reduced to 81%).
In addition, the size of individual metal contact pixels of contact
layer 530 is preferably kept as small as possible so that current
spreading from the individual contact pixels occurs. This increases
the probability that the generated photons will strike the
reflector 540 rather than the contact pixel of contact layer 530
(which would typically occur if the current traveled straight down
from the contact metal pixel of contact layer 530). The
polarization-doped AlGaN enables current spreading while
maintaining transparency even with a thin SPG layer 520; the thin
SPG layer 520 is used primarily to lower the contact resistance
while maintaining low absorption. This is in direct contrast to
conventional methods where p-doping in high Al content
Al.sub.xGa.sub.1-xN is highly resistive and will not allow current
spreading.
[0045] Embodiments of the invention may utilize photon-extraction
techniques described in the '093 application. Such techniques
include surface treatment (e.g., roughening, texturing, and/or
patterning), substrate thinning, substrate removal, and/or the use
of rigid lenses with thin intermediate encapsulant layers.
Exemplary substrate-removal techniques include laser lift-off, as
described in "High brightness LEDs for general lighting
applications using the new Thin GaN.TM.Technology", V. Haerle, et
al., Phys. Stat. Sol. (a) 201, 2736 (2004), the entire disclosure
of which is incorporated by reference herein.
[0046] In embodiments in which the device substrate is thinned or
removed, the back surface of the substrate may be ground, for
example, with a 600 to 1800 grit wheel. The removal rate of this
step may be purposefully maintained at a low level (approximately
0.3-0.4 .mu.m/s) in order to avoid damaging the substrate or the
device layers thereover.
[0047] After the optional grinding step, the back surface may be
polished with a polishing slurry, e.g., a solution of equal parts
of distilled water and a commercial colloidal suspension of silica
in a buffered solution of KOH and water. The removal rate of this
step may vary between approximately 10 .mu.m/min and approximately
15 .mu.m/min. The substrate may be thinned down to a thickness of
approximately 200 .mu.m to approximately 250 .mu.m, or even to a
thickness of approximately 20 .mu.m to approximately 50 .mu.m,
although the scope of the invention is not limited by this range.
In other embodiments, the substrate is thinned to approximately 20
.mu.m or less, or even substantially completely removed. The
thinning step is preferably followed by wafer cleaning in, e.g.,
one or more organic solvents. In one embodiment of the invention,
the cleaning step includes immersion of the substrate in boiling
acetone for approximately 10 minutes, followed by immersion in
boiling methanol for approximately 10 minutes.
[0048] Structures fabricated utilizing the above-described
techniques in accordance with various embodiments of the present
invention have been fabricated with three different reflector metal
areas, 0%, 51%, and 60%. No significant forward voltage increase
was observed at 51% reflector metal area with only 0.1 V increase
at 100 mA (while a 0.4 V increase was seen at 60% reflector metal
area), and an improvement in extraction efficiency of 24% was
measured for devices emitting through a thick absorbing AN
substrate with 51% reflector metal area. However, when combined
with die thinning, roughening, and encapsulation an overall gain of
.about.100% was achieved for devices with 51% reflector metal area
compared to devices with 0% reflector area. The results from 60%
reflector area were improved less than 51%, but optimization of
both contact metal spacing and reflector area may result in further
gains in overall efficiency.
[0049] The terms and expressions employed herein are used as terms
of description and not of limitation, and there is no intention, in
the use of such terms and expressions, of excluding any equivalents
of the features shown and described or portions thereof, but it is
recognized that various modifications are possible within the scope
of the invention claimed.
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